Lower strength material for MLS active layers

ABSTRACT

An improved material for use in a gasket includes a tensile strength in a range of about 1000 MPA to about 1150 MPA and a yield strength of at least about 90% of the tensile strength.

TECHNICAL FIELD

The present invention relates to multi-layer steel (MLS) gaskets and in particular to MLS gasket materials that exhibit improved sealing characteristics.

BACKGROUND OF THE INVENTION

The trends to reduce fuel consumption and emissions in internal combustion engine powered vehicles have placed increased demands on the performance of many components. Reducing fuel consumption by using lighter materials in engine cylinder blocks and head assemblies has proven successful, although the lighter alloys used typically experience greater deflection with equivalent cylinder compression ratios. This reduced stiffness may result in additional deflection within the head assembly and cylinder block, resulting in greater deflection between the head assembly and cylinder block, and thus, increased demand on a cylinder head gasket to accommodate relative deflection.

Reducing emissions by increasing the engine compression ratio has also proven successful. However, this increase in cylinder pressure typically results in increased motion between the mating surfaces of the head assembly and cylinder block. These factors, and others, have resulted in the technology of MLS cylinder head gaskets becoming an area of constant innovation.

The gasket areas immediately adjacent the circumference of engine cylinder bore apertures are subject to considerably greater stresses for assuring proper sealing than areas of the gasket radially remote from the apertures. These gasket areas immediately adjacent the circumference of engine cylinder bore apertures also experience greater displacement between the mating surfaces than areas of the gasket radially remote from the apertures. Typically, MLS gaskets incorporate at least one beaded region to ensure an adequate seal.

This displacement between the mating surfaces results in axial motion within the active layers and creates a micro-motion between the active layer and any adjoining surface. This motion requires a minimum durability in the beaded region to ensure that the gasket repeatably seals between the mating surfaces.

Layers with beaded regions, also called “active” layers, exert pressure on the sealing surfaces to ensure an adequate seal. Generally, the higher the surface pressures, the better the sealing function, or sealing capability. MLS gaskets sealing between a cylinder head assembly and a cylinder block are typically exposed to temperatures that often exceed 1600° F.

Selection of materials for the active layers is typically limited due to the desired hardness, durability, spring rate, ductility, softening point, and other characteristics. Conventionally, materials have been selected by considering the available desirable properties of materials that have evolved into use.

A typical material for active layers of MLS gaskets is 301 stainless steel (301 SS). Tensile strength for 301 SS is in a range of about 1350 to 1600 MPA, and yield strength is in a range of about 1050 to 1250 MPA. While 301 SS has the capacity to retain adequate properties during engine operation, improved materials are sought to increase gasket sealability, reliability, and/or capacity to withstand increased displacement between the mating surfaces.

SUMMARY OF THE INVENTION

The present invention relates to a material for a MLS gasket that has a yield strength that is greater than about 90% of the tensile strength. The tensile strength is in a range of about 1000 MPA to about 1150 MPA.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a partial view of a multi-layer steel cylinder head gasket in accordance with an embodiment of the present invention.

FIG. 2 is a sectional view of the gasket of FIG. 1, taken along line 2-2, with the layers separated for clarity.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an embodiment of a metal gasket 20 which is a cylinder head gasket. The gasket 20 is positioned between mating surfaces of a cylinder head assembly (not shown) and a cylinder block (not shown) of an internal combustion engine. The gasket 20 includes at least one metal layer 24. As depicted, only the uppermost metal layer 24 is shown. Each metal layer 24 is defined by a plurality of cylinder apertures 26, bolt apertures 28, and jacket apertures 30. Each jacket aperture 30 may transport a cooling fluid, or a lubricating fluid. The metal layers 24 are arranged such that the apertures 26, 28, 30 are generally aligned.

FIG. 2 illustrates gasket 20 to further include a second metal layer 32. Second metal layer 32 includes a first surface 36, a second surface 38, and a cylinder region surface 40. Second metal layer 32 is also illustrated to include a bead region 46 and a stopper region 48. As illustrated, second metal layer 32 extends in a radial direction R with bead region 46 and stopper region 48 having portions that extend in an axial direction A. Second metal layer 32 acts as an active layer, as described in greater detail below.

During installation of the gasket of FIGS. 1 and 2, bead region 46 is partially compressed in the axial direction A, thereby causing bead region 46 to foreshorten in the axial direction A and portions of bead region 46 to experience some movement in the radial direction R. During engine operation, relative movement between the mating surfaces in the axial direction A requires portions of bead region 46 to elastically move in the axial direction A in order to properly seal the mating surfaces. This elastic movement in the axial direction A of bead region 46 causes micro-motion of portions of bead region 46 in the radial direction R relative to surfaces in contact with first surface 36 and second surface 38. When bead region 46 does not have sufficient strength throughout bead region 46, cracking and fretting may occur.

In order to improve the desired physical properties of materials used in active layers, such as second metal layer 32, a theoretical simulation has been performed to identify desirable properties for bead region 46. Since many physical properties of metals vary as temperatures exceed several hundred degrees F., and some physical properties vary in relation to others, material selection is complicated by a variety of competing factors.

One of these competing factors is internal stress. In order to improve durability, internal stress should be reduced. However, in order to improve sealing function, internal stress should be increased to correspondingly increase the surface pressure that bead region 46 applies in axial direction A.

Computer simulations performed by the inventor for optimizing the physical properties of bead region 46 have indicated that a tensile strength (at about 70° F.) in a range of about 1000 MPA (145 ksi) to about 1150 MPA (166 ksi) and a yield strength of at least about 90% of the tensile strength will result in a gasket that is optimized for both durability and sealing function. Preferably, the material will retain about 70% of these respective strengths at gasket operating temperatures. These properties were determined by keeping the internal stress at a minimum value to ensure adequate sealing function.

Testing was performed on materials with a yield strength that is within 90% of the tensile strength, although these materials have a tensile strength below 1000 MPA. The durability of gaskets produced with these materials was adequate, but the sealing capabilities did not meet the desired test requirements. A material with an increase in both tensile strength and yield strength over the tested materials is expected to produce a gasket with desired durability and sealing capacity.

Preferably, materials for second metal layer 32 are stainless steels, including those that are austenitic, martensitic, and ferritic, although other materials, such as an Inconel® that maintains these desired properties at operating temperatures, may also be used.

While the invention has been described with respect to specific examples including preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims. 

1. A material for a multi-layer steel gasket, comprising: a predetermined tensile strength in a range of about 1000 MPA to about 1150 MPA; and a predetermined range of yield strength, wherein said yield strength is greater than about 90% of said tensile strength, wherein said tensile strength of about 1000 MPA to about 1150 MPA is measured at about 70° F., and said material has a tensile strength of greater than about 700 MPA at a maximum normal operating temperature of an internal combustion engine.
 2. The material of claim 1, wherein said material has a tensile strength of greater than about 700 MPA at about 1600° F.
 3. The material of claim 1, wherein said yield strength of greater than about 90% of said tensile strength is measured at about 70° F., and said material has a yield strength greater than about 630 MPA at about 1600° F.
 4. The material of claim 1, wherein said material is a stainless steel.
 5. The material of claim 1, wherein said material is an Inconel®.
 6. A multi-layer steel gasket comprising: a metal layer having a bead region, wherein at least a portion of said bead region has a predetermined tensile strength in a range of about 1000 MPA to about 1150 MPA, and a predetermined range of yield strength, wherein said yield strength is greater than about 90% of said tensile strength.
 7. The gasket of claim 6, wherein said tensile strength of about 1000 MPA to about 1150 MPA is measured at about 70° F., and said material has a tensile strength of greater than about 700 MPA at about 1600° F.
 8. The gasket of claim 6, wherein said yield strength of greater than about 90% of said tensile strength is measured at about 70° F., and said material has a yield strength greater than about 630 MPA at about 1600° F.
 9. The gasket of claim 6, wherein said bead region surrounds a cylinder aperture, and said gasket is selectively interposed between a cylinder head and a cylinder block of an internal combustion engine.
 10. The gasket of claim 9, wherein said bead region selectively moves relative to the cylinder block during operation of said engine.
 11. The gasket of claim 6, wherein all portions of said metal layer have a predetermined tensile strength in a range of about 1000 MPA to about 1150 MPA, and a predetermined range of yield strength is greater than about 90% of said tensile strength.
 12. The gasket of claim 6, wherein said metal layer includes a stopper region.
 13. The gasket of claim 12, wherein said stopper region circumscribes said bead region.
 14. The gasket of claim 6, wherein said metal layer includes a bolt aperture.
 15. The gasket of claim 6, wherein said bead region is formed of a stainless steel.
 16. A method of sealing an apparatus having a first component and a second component comprising: selecting a material for a bead region of a metal layer, wherein the material that has a predetermined tensile strength in a range of about 1000 MPA to about 1150 MPA, and a predetermined range of yield strength, wherein the yield strength is greater than about 90% of the tensile strength, wherein said step of forming the metal layer includes the step of forming the bead portion; forming the metal layer using the material; and positioning the metal layer between the first component and the second component.
 17. The method of claim 16, wherein said step of selecting includes the step of performing a theoretical simulation of the gasket in a desired environment to identify desired physical properties of the bead portion material, and wherein the desired physical properties of the bead portion material include tensile strength and yield strength.
 18. The method of claim 16, wherein the first component is a cylinder head for an internal combustion engine.
 19. The method of claim 16, wherein said step of forming the metal layer includes the step of forming a stopper portion on a portion of the metal layer.
 20. The method of claim 16, wherein the yield strength of greater than about 90% of the tensile strength is measured at about 70° F., and the material has a yield strength greater than about 630 MPA at about 1600° F. 